Add Streamlit state helpers for query parameters

[felipeimpieri]

Summary

• add a Streamlit UI state helper module for working with query parameters
• support both the new st.query_params API and legacy experimental APIs
• provide helpers to coerce parameter types and keep session state in sync

Testing

• python -m py_compile quantboard/ui/state.py

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Honor provided values when default is None

_coerce_type short-circuits whenever the default is None, returning None even if a query param or existing session-state value is present. Any get_param("foo", None) call will therefore overwrite a supplied foo query string or previously set session state with None, preventing optional parameters from ever being read. The helper should only fall back to None when the incoming value is absent or cannot be coerced, not unconditionally discard it.

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